1. Field of the Invention
The present invention relates generally to electron projection lithography (EPL), and more particular, to an apparatus and method for forming a mask blank having a single transition metal-based scattering layer with a final stress state substantially close to a desired final stress for use in lithographic mask formation in EPL systems.
2. Description of Related Art
In modern semiconductor development, semiconductors having decreased feature sizes are increasingly desired, particularly those having feature sizes of 100 nm or smaller. As conventional optical lithographic systems are limited to feature sizes of about 250 nm, one area of focus in developing such modem semiconductors has been to provide lithographic masks with continually decreasing feature sizes while still maintaining the speed, performance, and reliability of the resultant semiconductor.
Utilization of lithographic sources with shorter wavelengths such as x-rays and electron beams provide the potential to meet such feature size requirements of 100 nm or smaller. In x-ray and electron beam lithography (EPL) systems, masks such as membrane masks are required to meet the smaller feature size requirements. The use of membrane masks for x-ray and EPL systems are well known in the art. Typically, the masks are fabricated by depositing a thick absorber layer to a thickness ranging from 300 nm to 1000 nm, or a thin scattering layer for EPL systems to a thickness ranging from 30 nm to 50 nm over a substrate having a thin membrane layer on a surface thereof. The thin membrane layer typically comprises a highly doped silicon, silicon nitride, silicon carbide, diamond or materials as known and used in the art deposited to a thickness of 2-3 microns for use in x-ray lithography and to a thickness of 100-200 nm thick for use in EPL. The x-rays or electron beams pass through the thin membrane layer without significant diffraction or absorption loss.
Electron-beam projection lithography (EPL) systems are well known in the art of semiconductor formation such as SCALPEL and PREVAIL, for example. As illustrated in the prior art EPL mask of FIGS. 1A-B, a mask blank 10 may be provided for forming the mask over the substrate. As illustrated in FIG. 1A, the mask blank 10 may comprise a thin membrane layer 14, followed by an etch stop layer 16 and an overlying scattering layer 18, provided over a substrate surface 12. As will be recognized, the overlying scattering layer may be a multi-film scattering layer or a single film scattering layer.
As illustrated in FIG. 1B, once the absorber or scattering layer has been deposited over the membrane layer on the substrate, a membrane pattern is typically etched through the substrate from the backside using the membrane material as an etch stop, whereby the absorber or scattering layer is then patterned and etched to complete the mask. The absorption or scattering of incident energy by the patterned layer on top of the membrane then gives rise to the image on an underlying substrate coated with photoresist for subsequent device fabrication.
Typically, the scattering layer of the thin mask blanks used in EPL systems are formed over a membrane layer on the substrate by a multi-deposition process of first sputter depositing the etch stop layer over the membrane layer followed by deposition of a scattering layer thereover the etch stop layer. Typically, the etch stop layer comprises a thin chromium film while the scattering layer comprises a tungsten film. In depositing the etch stop and scattering layers of conventional mask blanks used in EPL systems, the multi-layer deposition processes generally require separate targets and separate deposition chambers or tools for the etch stop and scattering layers, whereby the substrate may be required to be moved from one chamber to the next. In depositing the etch stop and scattering layers, the as-deposited stress of the etch stop layer is typically tensile, i.e. positive, while the as-deposited stress of the scattering layer is typically compressive, i.e. negative, resulting in a combined film stack stress of the mask blank being either tensile or compressive. However, in order to obtain sufficient and reliable subsequent pattern placement on the finished mask, it is necessary that the mask blank for use in EPL systems have a combined film stack stress as close to zero as possible for subsequent mask finishing procedures.
In recognizing the above problem, prior art is aimed at controlling both stresses and thicknesses of both the etch stop and scattering layers of the mask blank during deposition procedures to produce a resultant combined film stack having a zero stress state for subsequent pattern placement on the finished mask blank to form a mask for use in EPL systems. However, conventional processes of forming EPL mask blanks also create scattering layer stacks with vertical regions of sharply different stresses, resulting in a final film stack stress of the mask blank varying significantly from substrate to substrate within a single lot of substrates, wherein some stacks possess unacceptable tensile or compressive stresses. As a result of some stacks possessing unacceptable tensile or compressive stresses, subsequent mask finishing procedures result in a final mask having reduced yield requiring additional fabrication steps as well as increased manufacturing times and costs to correct or produce an efficient mask for use in EPL systems.
In controlling the stress of the thick absorber films, prior art is further directed to controlling the stresses in thick absorber layers, such as those thick absorber films for use in x-ray mask, by re-annealing an absorber layer on a single substrate in a two-part annealing process. For example, a two step annealing process for developing and controlling stress in thick absorber films, such as those having thicknesses ranging from 300 nm to 1000 nm, may be used whereby a substrate having a thick absorbing layer is annealed in a first anneal step, the stress of the absorbing layer measured, and subsequently further annealing the same substrate to obtain a near zero stress state of the thick x-ray absorbing scattering layer.
As modern semiconductors continue to decrease in size, and therewith the thin scattering layers of mask blanks for use in EPL systems, a need continues to exist in the art to provide improved methods of making such thin scattering layers for use in the modern, smaller mask blanks for use in EPL systems, whereby the thin scattering layers have as-deposited internal stress states substantially near zero, or alternatively slightly tensile.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide an improved apparatus and method of forming a thin scattering layer for use in an improved mask blank for EPL systems whereby the improved thin scattering layer has a final stress state as near as possible to a desired stress, or stress range, such as stress states substantially close to zero, or alternatively slightly tensile.
It is another object of the present invention to provide an apparatus and method for eliminating the need for a multiple deposition and/or multi-layer deposition process for forming mask blanks for use in EPL systems.
Yet another object of the present invention is to provide a method of forming and a thin scattering layer having a high scattering cross section evident in high atomic number or highly dense materials.
It is another object of the present invention to provide an apparatus and method of forming a thin scattering layer having desirable etch characteristics.
Another object of the present invention is to provide a method of forming and a thin scattering layer having increased control of the final stress states of the thin scattering layer.
Still another object of the present invention is to provide a method of forming and a thin scattering layer whereby stress control is maintained from substrate to substrate in a single lot.
It is another object of the present invention to provide a method of forming and a thin scattering layer having near infinite adjustment of the final stress.
Another object of the present invention is to provide a method of forming and a thin scattering layer free from defects and foreign materials which could render a resultant, final mask useless in device manufacture.
Yet another object of the present invention is to provide a method of forming a thin scattering layer at decreased costs and time.
Still other objects and advantages of the present invention will in part be obvious and will in part be apparent from the specification.
The above and other objects and advantages, which will be apparent to one of skill in the art, are achieved in the present invention which is directed to, in a first aspect, a method for creating a scattering layer by providing a plurality of substrates, such as semiconductor substrates and silicon substrates, with a scattering layer and controlling stress in the scattering layer by subsequently annealing scattering layers over subsequently selected substrates from the plurality of substrates to an anneal temperature, and adjusting the anneal temperature based on a change in stress measure in the scattering layer over the subsequently selected substrates until the scattering layer stress is stabilized to a desired stress value. Membrane layers may also be directly provided thereover the plurality of substrates, wherein the membrane layers are in direct contact with the scattering layer.
The scattering layer of the present invention may be used in a mask blank for use in EPL. Preferably, the scattering layer is an amorphous or nanocrystalline structure with a high scattering cross section, and even more preferably, the scattering layer is a transition metal-based scattering layer having a stoichiometry of TaxSi selected from the group of transition metals having atomic numbers consisting of 21-30, 39-48 and 57-80 over the plurality of substrates. Preferably, the stoichiometry of the present transition metal-based scattering layer is Ta2Si.
The thin transition metal-based scattering layer of the present invention may further include at least another transition metal selected from the group of transition metals having atomic numbers consisting of 21-30, 39-48 and 57-80.
In another aspect of the present invention, the thin transition metal-based scattering layer of the present invention is a binary composite having the molecular formula AxBy where A comprises the transition metal, B comprises an element selected from the group of elements consisting of Sc, To, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, La, Hf, Ta, W, Re, Of, Ir, Pt, Au, Hg, B, Al, Ga, In, Tl, C, Si, Ge, Sn, Pb, N, P, As, Sb, Bi, O, S, Se, Te, and Po, and x and y are real numbers representing a number of atoms of each A and B.
In yet another aspect of the present invention, the thin transition metal-based scattering layer of the present invention is a ternary composite having the molecular formula AxByCz where A comprises the transition metal, B and C each comprise an element selected from the above group of elements, and x, y and z are real numbers representing a number of atoms of each A, B and C.
In still another aspect of the present invention, the thin transition metal-based scattering layer of the present invention may further include oxygen, nitrogen, and combinations thereof.
Preferably the present scattering layer is deposited to a thickness ranging from about 30 nm to about 50 nm by techniques including DC magnetron sputter, DC bipolar sputter, and AC sputter. In depositing the scattering layer, the scattering layers are provided over the plurality of substrates by adjusting deposition parameters to provide reproducible, uniform scattering layers over the plurality of substrates.
Subsequently, the stress of the present scattering layer is controlled by annealing subsequent scattering layers over subsequently selected substrates from the plurality of substrates to an anneal temperature, and adjusting the anneal temperature based on a change in stress measure in the scattering layer over the subsequently selected substrates until the scattering layer stress is stabilized to a desired stress value. In the preferred embodiment, in controlling the stress of the scattering layer by annealing, the anneal temperature may be adjusted to temperatures ranging from about 300xc2x0 C. to about 500xc2x0 C. The scattering layers may be annealed for a time ranging from about 5 minutes to about 20 minutes. Preferably, the scattering layer is annealed by a vertical tube furnace rapid thermal annealing system. Once the final anneal temperature is determined, all remaining substrates of the plurality of substrates may be annealed at the final anneal temperature to provide scattering layers over remaining ones of the plurality of substrates with the desired internal stress state. Preferably the final film stress is controlled to within xc2x110 Mpa of the targeted stress.
The present invention further includes allowing the substrates having the scattering layers at the desired internal stress state to equilibrate until an internal stress of the scattering layers are stabilized to a desired target.
In a further aspect of forming the present scattering layer, a first substrate stress is determined by measuring a first substrate bow, cleaning a surface of the substrate, providing the scattering layers over the cleaned substrate surface, measuring a second substrate bow, and then determining the first substrate stress by calculating a difference between the first substrate bow and the second substrate bow. Alternatively, the surface of the substrate may be cleaned before the first substrate bow is measured.
In the preferred embodiment, the present invention provides a method for creating a scattering layer by providing a plurality of substrates, providing scattering layers thereover the substrates, determining a desired internal stress state of the scattering layers, and then controlling stress in the scattering layers over the plurality of substrates by the following steps:
a) annealing a first substrate from the plurality of substrates having the scattering layers thereover at a first anneal temperature;
b) measuring an internal stress of the scattering layer over the first substrate;
c) determining a second anneal temperature based on the internal stress of the scattering layer over the first substrate;
d) annealing a second substrate from the plurality of substrates at the second anneal temperature;
e) measuring an internal stress of the scattering layer over the second substrate; and
f) repeating steps a)-e) until a final anneal temperature is determined at which the stress in the scattering layers is at the desired internal stress state of the scattering layers.
In such an embodiment, once the final anneal temperature has been determined, all remaining ones of the plurality of substrates at the final anneal temperature to obtain scattering layers over the remaining ones of the plurality of substrates having the desired internal stress state may be annealed at the such final temperature.
In another aspect, the present invention provides a method for creating a mask blank comprising providing the plurality of substrates; providing a membrane layer over the plurality of substrates; and then providing scattering layers over the plurality of substrates, the scattering layers having stress controlled by subsequently annealing scattering layers over subsequently selected substrates from the plurality of substrates to an anneal temperature and adjusting the anneal temperature based on a change in a stress measure in the scattering layer over the subsequently selected substrates until the stress in the scattering layer is stabilized to a desired stress value. Preferably, the membrane layer comprises a material selected from the group consisting of silicon nitride, silicon carbide, and diamond, deposited to a thickness of about 50 nm to about 200 nm.
In yet another aspect, the present invention provides a mask blank for electron projection lithography comprising a substrate, a membrane layer directly over the substrate, and a transition metal-based scattering layer directly over the membrane layer, the transition metal-based scattering layer having an internal stress at a desired stress value.